It is known from British Patent No. 2159998B to detect and store a time varying characteristic, e.g. the stress wave activity produced in a body by a writing instrument during the generation of a given piece of handwriting. The time varying characteristic, e.g. the stress wave activity, produced by a subsequent generation of the given piece of handwriting is compared with that produced by the stored piece of handwriting in order to determine whether the stored and subsequent handwriting pieces have a common author.
In our copending European Patent Application No. 90313505.1, filed on 12 Dec. 1990, if it is determined that the subsequent and stored time varying characteristic produced during the generation of the handwriting pieces have a common author, a weighted average of the stored time varying characteristic and the subsequent time varying characteristic is produced. The stored time varying characteristic is then replaced by the weighted average of the stored time varying characteristic and the subsequent time varying characteristic produced during the generation of a piece of handwriting by a common author. The weighted average is a weighted average of the magnitudes of the time varying characteristic between the stored time varying characteristic and the time distorted subsequent time varying characteristic and is a weighted average of the time of events.
In this procedure it is necessary that the stored time varying characteristic is either produced from a single sample piece of handwriting of the author generated during an enrollment procedure, or is an average produced from a number of sample pieces of handwriting of the author generated during an enrollment procedure.
A problem associated with the production of the stored time varying characteristic is that the single sample piece of handwriting of the author, or one or more of the number of sample pieces of handwriting of the author, is often not a typical piece of handwriting of the author. This is often because the author is not relaxed during the enrollment procedure, or is because the author is not used to writing or for other reasons. If the time varying characteristic produced from the untypical sample of the piece of handwriting, or untypical samples of the piece of handwriting, is stored and subsequently compared to the time varying characteristic produced by an authentic piece of handwriting it is possible that the authentic piece of handwriting will be determined to be generated by a person other than the author of the stored time varying characteristic, i.e the piece of handwriting will be determined to be a forgery.